In the field of MR imaging the demand for improved medical diagnosis has led to the development of so-called combined-modality imaging assemblies. The combination of an MR imaging system with a nuclear imaging system improves diagnosis by augmenting the soft-tissue image contrast benefits of MR with the functional imaging capabilities of the nuclear imaging system. The nuclear imaging system is typically a PET or a SPECT imaging system and may for example be a whole-body or a pre-clinical imaging system. However the design of combined-modality imaging assemblies is frustrated by interoperability constraints. The few-Tesla magnetic fields and the high RF fields generated within the MR imaging system bore restrict the design freedom in the nuclear imaging system, limiting for example the range of materials that can be used. Furthermore the operation of the two imaging systems in close proximity risks interference from the nuclear imaging system degrading the MR image quality.
Such combined imaging assemblies may be formed through co-location, in which the nuclear imaging system is positioned close to the MR imaging system. During operation, a transfer mechanism such as a patient support pallet is translated between the two imaging systems and the MR and nuclear images are acquired consecutively. The separation between the imaging systems relaxes the impact of one system on the other but risks patient motion between the consecutive acquisitions degrading image quality. Such combined imaging assemblies may also be fully integrated, in which an MR imaging system is combined with a nuclear imaging system in the same housing offering both simultaneous acquisition and a reduction of image artefacts at the expense of aggravated interoperability issues.
A particular interoperability issue found when an MR imaging system is combined with a nuclear imaging system is that of electrical interference between the nuclear imaging system and the MR imaging system. In this, electrical currents flowing in the circuits of the nuclear imaging system produce electromagnetic radiation which risks being detected by the sensitive RF sense coils in the MR imaging system. The problem is particularly acute in simultaneous-acquisition assemblies in which a shared imaging region necessarily requires some parts of the nuclear imaging system to be located close to the bore of the MR imaging system where the sensitive RF coils are located. The RF sense coils are typically sensitive to a particular frequency bandwidth, thus only frequencies within this bandwidth present an issue. However, digital signals commonly used in the nuclear imaging system have an inherently broad RF emission spectral bandwidth which may therefore fall within the detection bandwidth of the MR RF receive coil and thereby interfere with the MR imaging system.
Conventional methods of reducing such interference include the use of electrical screening of the interference-generating regions. Whilst effective, a drawback of their use close to the bore of the MR imaging system is that electrical screening materials can distort the MR magnetic field and thereby degrade the MR image quality. More specifically, eddy currents induced in such a conductive screen soften the time profile of the switching gradient which can lead to a distortion in MR k space.
Other techniques for reducing interference in combined imaging assembly are disclosed in patent application US2009/0195249A1. These include the spectral separation of the RF interference from the magnetic resonance frequency by for example the use of clocking frequencies, and or power supply switching frequencies that are not at the magnetic resonance frequency, or by use of such frequencies which do not have harmonics at the magnetic resonance frequency. The use of notch filters centered at the magnetic resonance frequency and improved shielding is also disclosed in patent application US2009/0195249A1.
Whilst the above-mentioned approaches go some way to reducing interference between a nuclear imaging system and an MR imaging system, the demand for improved quality MR images to further improve patient diagnosis requires this interference to be reduced even further.